During the past year we have examined macromolecular synthesis in developing red blood cells of chicken embryos, a system which provides populations of erythroblasts homogeneous in terms of cell cycle when taken from embryos at an early stage and a similarly homogeneous, but now functionally distinct, population when taken from an older embryo. We have determined optimal cell and precursor concentrations for the synthesis of both protein and RNA and have studied the effect of pool size on the uptake and utilization of RNA precursors. We have found small but reproducible differences in the rates of synthesis of RNA and protein and have determined that precursor availability may be differentially important to the two cell types. RNA synthesized by the cells has been analyzed by gel electrophoresis; several minor species have been found and are being examined for messenger RNA characteristics. We are currently extending these studies to the macromolecular synthesis occurring during the various stages of a given cell cycle.